Diesel engine generator, selection of engine and alternator 1

[thongwathar]

Can somneone answer me following requirements for diesel engine generator:
1) engine must have a power margin of 10 % above the power that is required to produce required power at generator terminal
2) the power versus temperature curve for generator must be above that of the engine for all range of site ambient temperatures

The required power at generator terminal is 630 KVA( 504 ekW ). As per requirement 1, engine must be selected at least to have 554 ekW( i.e. 504 x 1.1 ).

Then as per requirement 2, an alternator of rated output above 554 ekW must be selected.
I hope that comply above two requirements.

 

[waross]

These issues and more such as dynamic balancing issues are addressed by the manufacturers of gen sets.
The engine power of a STANDBY set is adequate for the required output. The margin may be less than 10%
The engine power of a PRIME POWER set is 25% larger.
I see many sets that are dual rated for both prime power and standby power.
Temperature curves. Typically the air flow is in the rear of the alternator and out the front. Then across the engine, back to front and through the radiatr. The engine has a "Pusher" fan so that the air flow is over the engine and then out through the radiator and hopefully discharging outside. The coordination of cooling curves is addressed by the manufacture of the complete set.
Typically gen sets are rated at 80% Power Factor.
A 625 KVA set will be rated at 500 KW and the engine sized on the basis of 500 KW.
Before you waste a lot of time trying to designe and build a set, I suggest that you compare the prices of complete Gen-sets with the price of the components to build your own.
Add the price of the transfer switch as well. When a transfer switch is included in the purchase of a gen-set the price adder is very competetive.
Check the Onan web site, the Olympian web site, and any other manufacturers sites you can google up. They will give you set sizes and ratings including engine model numbers.
yours
yours

 

[taylorg]

With respect to point 1, are you referring to the requirement of ISO-3046 for 10% 'overload'? This requires the engine to be designed and rated to be able to withstand running at 110% load for 1 hour in every 12.

 

[waross]

Hello taylorg
Mu understanding is that the 10% rule that you mention applies to prime power sets, not standby sets.
The 25% was common on older prime power sets, even though the 10% rule applies. The extra margin afforded by the 25% allows the set to produce full output when the set is old and worn and ready for a rebuild.
Standby sets are often rated at the absolute maximum. The 10% mentioned above may be added to the rating, and no overload is allowed. What I mean is that a prime power set rated at 500 KVA Prime power may be re-rated at 550 KVA Standby. The engine may be rated at 100% rather than 125%.
The 10% that I mentioned was based on a couple of incidents in the field when a set would produce rated power but could not sustain even a few percent overload.
For a complete understanding I would suggest browsing some Generator Manufactutrer site and comparing similar sets.
F.G. Wilson, Caterpilar- Olympian line or Marathon line,
Onan.
You can usually identify similar sets by similar model numbers with suffix codes to indicate prime or standby.

 

[taylorg]

waross,

Unless I am mistaken, Thongwathar didn't actaully specify whether he was asking about a standby set or baseload generator. Perhaps you can enlighten us Thongwathar?

 

[thongwathar]

Dear All,

Thank a lot for all you responses. It is for a standby gen set.
I had now an explanation as below from someone for how to size a stanby gent set.

The required output power specified is 630 KVA ( 504 eKw with 0.8 p.f ). Assuming 95 % generator generator effiency,the required engine power is 531 Kw. With power margin of 10%,an engine of 585 Kw can be selected.

Then a generator of rated power = or > than 585 Kw ( 732 KVA )must be selected.

Hope you all agree to his explanation.

 

[waross]

One point to remember, thongwathar
A generator will not support an overload as well as a transformer.
If a transformer is over loaded it will run hotter. This will result in an eventual reduction in the life of the transformer. This is not instantaneous, it may take 15 or more minutes for the temperature to reach a stable temperature when the load is increased.
If a generator is loaded past the capability of the motor, it slows down, now, no 15 minutes. What happens next depends on how the load responds to under frequency. Most voltage regulators on small sets have Under Frequency Roll Off (UFRO) and reduce the voltage when the frequency drops.
It is well to be generous when sizing a generator.
You may not find a set rated at 630 KVA. Typically you would use the next larger available size of alternator and size the motor to the alternator rather than the load.
Eg 700 KVA, Alternator. (Check what is actually available)
With your assumptions, 700 KVA x .8 PF / .95 Eff. x 110% = 650 (648) eKw motor power.
Alternator = 700 KVA
Motor power 650 (648) eKw.
Another factor that is addressed by the set manufacturers is fuel quality. In Canada there can be a 10% difference in the specific gravity of diesel fuel. Winter diesel vs. summer diesel. When selecting the engine, it is well to check what specific gravity of fuel the ratings are based on.
You haven't mentioned voltage.
KVA is by definition Volts times Amps divided by 1000. If the Amps are exceeded the set will overheat. That is a limit.
The volts are somewhat arbitrary. As long as you don't approach flux saturation you can set the volts to what you need. A typical alternator for North America will be rated at a multiple of 120 volts.
Now, if you select a set rated at 700 KVA at 240 volts The current rating will be 700,000/240 or 2917 Amps.
The KVA ratings on the same alternator will be;
240 volts 700 KVA
230 volts 671 KVA
220 volts 642 KVA
208 volts 607 KVA
This is a factor that must be checked even when buying a complete set.
yours